UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Reply
 
Thread Tools
Old 9th Oct 2020, 12:43 pm   #1
Mikey405
Octode
 
Mikey405's Avatar
 
Join Date: Jul 2006
Location: Solihull, West Midlands and Beaford, Devon
Posts: 1,579
Default Poorly Philips A4 Chassis (AKA Pye 717)

Hi all.

After spending many man-days on a Philips G18C571/01 (Basically a Pye CT200/1 in a small G8-a-like cabinet with a few very slight differences to the circuit), Tas (of this forum) and I are rather stumped by an odd line-phase problem.

The picture is shifted on the raster a long way to the right. (The attached image has the brightness turned up and the contrast turned down in order to show where the raster starts on the left hand side.)

There is a "line phase" control on these sets which alters the "Pulse Width" circuit inside the TBA920 line oscillator chip but this control has very little effect on the picture - in fact it has virtually no effect whatsoever until the control is turned almost fully clockwise, and then the oscillator stops oscillating.

The voltages around the chip are mostly about 0.8 volts high (measured with a DMM) compared with those shown on the diagram.

Interestingly, the voltage on pin 3 (the phase control) is about 2 volts too high. If R614 (3K3) is increased in value to 4K7 then the phase control has more effect and the picture can be centred (the voltage on pin 3 is then roughly correct). This, however, isn't really cricket - and with the component changed to the wrong value, there is a "wobble" on the verticals, which isn't there with the correct value resistor.

The feedback from the line output transformer is fed directly from winding 664 through R512 to the chip and there's nothing much which might alter its phase. There is some kind of phase-changing network, being C610, R611 and C609 which are fed from the sync separator portion of the IC and then fed back into the line oscillator phase detector via pin 6. These components have all been replaced without effect.

All the waveforms around the IC look exactly like those in the manual, the chip itself has been replaced along with almost all directly-connected components - especially any which may have an effect on the phase component of the line oscillator - and still the fault eludes us.

So... Does anyone have any ideas?

Answers should be addressed to Tas, as the set is now back down with him - although I shall be following with interest.

Thanks all.

Kind regards.

From Mike.
Attached Thumbnails
Click image for larger version

Name:	IMG_20201009_114559.jpg
Views:	191
Size:	68.7 KB
ID:	217412   Click image for larger version

Name:	IMG_20201009_114703.jpg
Views:	148
Size:	72.0 KB
ID:	217413  
__________________
G7TRF
Mikey405 is offline   Reply With Quote
Old 9th Oct 2020, 12:50 pm   #2
Welsh Anorak
Dekatron
 
Welsh Anorak's Avatar
 
Join Date: Apr 2005
Location: North Wales, UK.
Posts: 5,072
Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

You don't fool us with your A4 nonsense - it's still a Pye CT200!
Although you've replaced the IC, ISTR the mono 320 did do this when the wrong spec IC had been fitted. I think it had some sort of suffix on it. However I'm clutching at the same straws you've been clutching at, I suspect.
__________________
Glyn
www.gdelectronics.wales
Welsh Anorak is offline   Reply With Quote
Old 9th Oct 2020, 7:13 pm   #3
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 5,875
Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

As an alternative thought...could there be some sort of DC shift on the scan coils?
__________________
On Spike Milligan's headstone......I told you I was ill...
Sideband is online now   Reply With Quote
Old 9th Oct 2020, 10:11 pm   #4
Mikey405
Octode
 
Mikey405's Avatar
 
Join Date: Jul 2006
Location: Solihull, West Midlands and Beaford, Devon
Posts: 1,579
Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

Hi Glyn and SB.

Re the Philips "A4" thing - I thought you'd like that. And regarding the TBA chip. that would have been an excellent suggestion but for the fact that it was the original device fitted to the set.

Tim J. (Studio 263) of this forum had an interesting suggestion re the sync separator triggering on the burst rather than the sync pulse so a check of the video DC levels into the chip may reveal something.

Regarding DC in the scan coils, the raster itself is correctly centred, SB, it's just the picture within the raster. I think they used to call it "False Line Lock" in old books.

Thanks all.

Kind regards.

From Mike.
__________________
G7TRF
Mikey405 is offline   Reply With Quote
Old 9th Oct 2020, 11:17 pm   #5
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 6,330
Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

Isn't there a line phasing circuit in the TBA920, essentially a picture shift control? I can remember having this fault in the Invicta version of this set. Need to look up the internal architecture to find the phasing part.

DFWB.
FERNSEH is online now   Reply With Quote
Old 10th Oct 2020, 12:46 am   #6
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 6,330
Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

A topic about the Philips 570 was started in January 2018 by forum member Tas:

https://www.vintage-radio.net/forum/...d.php?t=142998

DFWB.
FERNSEH is online now   Reply With Quote
Old 10th Oct 2020, 9:58 am   #7
Welsh Anorak
Dekatron
 
Welsh Anorak's Avatar
 
Join Date: Apr 2005
Location: North Wales, UK.
Posts: 5,072
Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

Is there colour? That might point somewhere, or then again it might not. Having a look under the bonnet of the TBA might reveal something unexpected.
On an old TV you'd be looking at the flywheel diodes, so something must be upsetting that part of the circuit. Bearing in mind the iconic IF strip, could the video signal be distorted in a very strange way? More straws, I'm afraid...
__________________
Glyn
www.gdelectronics.wales
Welsh Anorak is offline   Reply With Quote
Old 18th Oct 2020, 7:19 pm   #8
Tazman1966
Nonode
 
Tazman1966's Avatar
 
Join Date: May 2006
Location: St Albans, Herts, UK.
Posts: 2,109
Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

Hello Mike, Glyn, Rich, and David.

Thanks for the replies. Unfortunately, I have been side tracked into looking at a load of my transistor radios that I have ben toying with the idea of getting shot of. The reason being that I had no room to even wok on this little tv so no updates at the moment. I'm determined to get to the bottom of the fault though so it's not forgotten!
__________________
All the very best,
Tas
Tazman1966 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 10:21 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.